1. Field of the Invention
This invention relates to a light emitting diode, more particularly to a light emitting diode with an optical adhesive present between an epitaxial film and a permanent substrate.
2. Description of the Related Art
Referring to FIG. 1, a conventional vertically structured light emitting diode (LED) 1 includes a permanent substrate 11, an epitaxial film 12 disposed on the permanent substrate 11 and generating light when electricity is supplied, and a top electrode 14 disposed on the epitaxial film 12.
The permanent substrate 11 is electrically conductive and the electricity may be supplied to the epitaxial film by virtue of electrical connection between the permanent substrate 11 and the top electrode 14. The permanent substrate 11 includes a base plate 111 and a reflective film 112 formed between the base plate 111 and the epitaxial film 12 for reflecting light.
The epitaxial film 12 is formed by growing epitaxially a gallium nitride series semiconductor material on an epitaxial substrate (not shown), and then is transferred to so as to be in ohmic contact with the permanent substrate 11 using a temporary substrate (not shown). The epitaxial film 12 includes p-doped and n-doped cladding layers 121,122, and an active layer 123 that is formed between the p-doped and n-doped cladding layers 121,122. A band gap exists between the p-doped and n-doped cladding layers 121,122. When electricity is supplied to the epitaxial film 12, recombination of electron-hole pairs occurs in the active layer 123, thereby releasing energy in a form of light. In order to improve the light emitting efficiency of the LED 1, an upper surface 124 of the epitaxial film 12 (i.e., an upper surface of the n-doped cladding layer 122) is roughened so as to reduce full reflection of the light generated from the epitaxial film 12. As a result, high proportion of the light generated from the active layer 123 can be directly emitted out of the LED 1 through the upper surface 124.
During production of the LED 1, the gallium nitride series semiconductor material is epitaxially grown on the epitaxial substrate, followed by doping to form the epitaxial film 12. Thereafter, the permanent substrate 11 is wafer-bonded to the epitaxial film 12, and the epitaxial substrate is removed to expose the upper surface 124 of the epitaxial film 12, followed by roughening the upper surface 124 and disposing the top electrode 14 on the roughened upper surface 124 to obtain the LED 1.
In the conventional LED 1, because the epitaxial film 12 and the permanent substrate 11 are connected only by wafer bonding, the connecting surfaces between the epitaxial film 12 and the permanent substrate 11 should be sufficiently flat so that the epitaxial film 12 is tightly bonded to the permanent substrate 11. Furthermore, in order to ensure a uniform ohmic contact between the permanent substrate 11 and the epitaxial film 12, the wafer bonding process is required to be conducted at a temperature ranging from 200° C. to 400° C. As a result, the reflective film 112 of the permanent substrate 11 may have a degraded quality on account of the high temperature. Accordingly, the reflectivity of the reflective film 112 may be adversely affected and the light emitting efficiency of the LED 1 may be lowered.